Using appropriate genetic procedures for the production of isogenic strains of E. coli and appropriate media for positive and negative controls, we have successfully analyzed the selective effects of four alleles determining naturally occurring allozymes of the enzyme 6-phosphogluconate dehydrogenase. The alleles are selectively neutral in the K-12 genetic background. We now propose to carry out similar studies of loci associated with a number of other allozyme polymorphisms, including alcohol dehydrogenase, glutamic oxaloacetate transferase, glucose-6-phosphate dehydrogenase, phosphoglucose isomerase, glyceraldehyde-3-phosphate dehydrogenase, peptidase, mannose phosphate isomerase, isocitrate dehydrogenase, and phosphoglucomutase. Possible multiple locus effects on fitness will also be examined, as will certain biochemical characteristics of allozymes found to be associated with selection. These studies bear on the hypothesis of selective neutrality as it pertains to allozymes.